. Bulletin. Natural history; Natuurlijke historie. 102 PEABODY MUSEUM BULLETIN 38 stock. It is possible that the main part of the stock developed through Rhinodip- terus ulrichi. (For a discussion of R. ulrichi and R. secans, see p. 37). The third main stock is represented by Rhinodipterus secans. This has retained many of the characters of Dipterus that are lost, or partly lost, in the other two, , bones D, O, and K and the continuation of the lateral-line canal from K to J. On the other hand it has a posteriorly projecting I and a long posterior parasphe- noidal stalk, differentiating it
. Bulletin. Natural history; Natuurlijke historie. 102 PEABODY MUSEUM BULLETIN 38 stock. It is possible that the main part of the stock developed through Rhinodip- terus ulrichi. (For a discussion of R. ulrichi and R. secans, see p. 37). The third main stock is represented by Rhinodipterus secans. This has retained many of the characters of Dipterus that are lost, or partly lost, in the other two, , bones D, O, and K and the continuation of the lateral-line canal from K to J. On the other hand it has a posteriorly projecting I and a long posterior parasphe- noidal stalk, differentiating it from Dipterus. The only known material occurs in the lower Famennian and is thus coeval with Fleurantia and Scaumenacia. The importance of this stock is that it provides the only possible origin for the Early Carboniferous and later genera (all of which retain D) unless one is prepared to admit the possibility of the redevelopment of a bone in the D position in the Scaumenacia stock. This we do not accept. Two stocks are present in the Lower Carboniferous—the Ctenodus stock and the Sagenodus stock. The former is characterized by relatively large paired C and E with a smaller D at the angle of junction of these four bones. In the latter stock C has atrophied and the vacated space is occupied by a forward extension of B and an enlargement of D. Both exhibit a number of features absent from all Devonian forms: with the doubtful exception of Conchopoma all genera show the capture of the occipital cross commissure by B and the incorporation of Z in the skull table. The parasphenoid, in all genera in which it has been observed, namely Sagenodus, Conchopoma, Uronemus, and Ctenodus, has a flatter rather massive anterior rhombic portion that lacks a continuation of the groove present on the Triassic Permian Upper Carboniferous Lower Carboniferous Upper Devonian Middle Devonian Lower Devonian Paraceratodus Conchopoma. Fig. 35. Phyletic relations of known Paleozoic Dipnoi in the light
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